Material based on copper zinc tin sulfide (CZTS) has been investigated to obtain its optimum performances due to the non-toxicity, elemental abundance, and large production capacity. In this work, fabrication of CZTS-based thin-film solar cells was carried out using an all-solution-non-vacuum-based method and spin-coating process for layer deposition. Additionally, the ZTO buffer layer was used as an alternative buffer layer instead of other materials such as CdS. The bandgap energies of the CZTS absorber layer and ZTO buffer layers are ~1.5 eV and ~3.7 eV, respectively. The solar cell fabricated by the all-solution process using the ZTO buffer layer exhibited the power conversion efficiency of 0.28%.